Samples, such as mineral samples and other work pieces, are analyzed using a variety of different techniques. In electron backscatter diffraction (EBSD), an electron beam dwells at a point on the work piece surface. Electrons that are backscattered near the surface are diffracted after scattering off crystal planes in the sample. The backscattered electrons are detected to form an image of a diffraction pattern. The diffraction pattern can be indexed and used to identify the crystal structure of the work piece at the dwell point. From the diffraction pattern, the crystal structure of the work piece at the dwell point can be determined. When an investigator has some idea of the materials that make up the work piece, the investigator can match the crystal structure determined by EBSD with the known crystal structures of the expected materials to determine the material's structure, phase, and/or composition at the dwell point.
Another technique for determining the composition at a dwell point is x-ray spectroscopy, including wavelength dispersive x-ray spectroscopy (WDS) and energy dispersive x-ray spectroscopy (EDS). In these method an electron beam is directed toward a sample and excites inner shell electrons of the sample. The decay from the excited state back to a lower energy state yields an x-ray with a energy/wavelength characteristic of the atom from which it was emitted. In EDS, the energies of x-rays coming from the sample in response to the electron beam are measured and plotted in a histogram to form a spectrum. The measured spectrum can be compared to the known spectra of various elements to determine which elements and minerals are present. Alternatively, the relative number of x-rays within specific frequency ranges or channels are compared to the relative number of x-rays within those same frequency ranges in reference materials to identify the material at the dwell point.
EDS can be used to determine the materials present in a work piece and then the crystal structures of the known materials can be compared to the crystal structure at individual dwell points as determined by EBSD or other techniques.
To map a surface by EDS or EBSD is time consuming. These techniques require relatively long dwell times at each dwell point to achieve good signal-to-noise ratios for obtaining high resolution grain and/or compositional images of the sample. To measure sufficient points to obtain a high resolution surface mapping requires measuring many dwell points that are close together. Obtaining sufficient x-rays for an individual dwell point can take 1 or 10 milliseconds. For an image of one thousand by one thousand pixels, obtaining the compositional information from the x-ray detector can take from about fifteen minutes to a few hours.
In a process known as “slice-and-view,” a three-dimensional image can be obtained by repetitively mapping a succession of two-dimensional surfaces, repetitively removing a thin slice to expose a new surface, and then mapping the newly exposed surface. The multiple two-dimensional maps are then mathematically combined to reconstruct a three-dimensional representation. The thin slices can be removed, for example, by a focused ion beam, a laser beam, or a microtome. The thickness of the thin slice can be between the order of nanometers, 10s of nanometers, 100s of nanometers or microns depending upon the slicing method and desired resolution. A laser allows a large volume to be removed with each slice resulting in a much larger surface area or “face” to be analyzed. The large surface area increases the time to map each surface, rendering high resolution mapping by EDS or EBSD impractically slow. For example, a slice-and-view surface prepared by FIB, plasma FIB, or laser may have an area greater than 5×5 microns square, greater than 50×50 microns square, or even greater than 1,000×1,000 microns square.
The surface can be mapped more quickly by measuring fewer dwell point by spacing the dwell points further apart or by reducing the dwell time, but the resulting image is coarse or fuzzy because insufficient data is acquired to produce a high resolution image.
The MLA Automated Mineral Analysis System from FEI Company, the assignor of the present invention, can operate in a rapid acquisition mode. An image is first acquired using a backscattered electron detector. The image is then processed to identify regions that appear from the contrast to have the same elemental composition. The beam is then positioned at the centroid of each identified region for a longer dwell time to collect an x-ray spectrum representative of the region. Similarly, U.S. Pat. Pub. 20130015351 to Kooijman et al, for “Clustering of Multi-Modal Data” describes directing an electron beam toward a work piece to determine grain boundaries and then combining x-ray data from points inside each grain to compile an x-ray spectrum.
Backscattering of electrons depends on the atomic number of the elements in the surface and upon the geometric relationship between the surface, the primary beam, and the detector. The backscattered electron image therefore shows boundaries between regions of composed of materials having different average atomic numbers and regions separate by topographical feature. Grains composed of the same or similar materials but having different crystal orientation may therefore be indistinguishable in a backscatter electron image. Grains composed of materials having similar average atomic numbers may also be difficult to identify.
FIGS. 1A-1C illustrate an EBSD mapping process. FIG. 1A shows a scanning electron microscope image 100 of a surface of a work piece. The surface may be, for example, the top surface of a polished mineral sample or a sample surface exposed by a FIB or laser slicing off a layer of the work piece. FIG. 1B shows an EBSD map 102 of the region shown in FIG. 1A. Dwell points represented by the same color indicate diffraction patterns showing similar characteristics. For example, all blue dwell points 108 represent the same first grain orientation, all red dwell points 112 represent the same second grain orientation, and all green dwell points 116 represent the same third grain orientation. Because each dwell point requires a relatively long time to obtain an EBSD image, fewer dwell points can be used in practice, so the dwell points of the EBSD map are relatively far apart. FIG. 1C shows the relatively low resolution grain orientation map 106 that results from the dwell points of FIG. 1B, with region 110 being defined by the dwell points 108, region 114 being defined by the dwell points 112, and region 118 defined by dwell points 116. Note how the map does not follow the contour of the electron microscope image exactly. The grain boundaries are similarly inexact.
When an ion beam is directed toward a work piece, the ions in the beam may be channeled into a crystal to varying degrees depending upon the angle that the ions make with the crystal planes. The number of secondary electrons emitted sufficiently close to the surface and available to be detected will vary with whether the ions are stopped near the surface or whether they channel deeper into the work piece. Secondary electrons created below the work piece surface are reabsorbed in the sample and are unlikely to escape the sample to be detected. Thus, crystal grains having different crystal orientations may show different gray levels on a scanning ion microscopy (SIM) image because the ion channeling depends on the orientation of the crystal planes relative to the beam. The term “ion channeling image” is used to mean a SIM image formed using the number of secondary electrons collected at each dwell point of the focused ino beam.
Using an ion beam for imaging and analysis has several disadvantages. The resolution of a SIM image is typically worse than the resolution of a scanning electron microscope (SEM) image. The mass of the ions will inevitably result in some damage to the sample. While a lower beam energy reduces the sample damage, the lower beam energy slows ions, providing more time for the ions in the beam to repel each other and spread out the beam, reducing resolution. Also, the ions implanted into the work piece can affect some of the properties of the work piece.
Another technique for determining crystal plane orientation entails directing an ion beam toward the work piece at different angles, and recording the number of secondary electrons (which correspond to the brightness of the SIM image) emitted at various angles. The relationship between the angle and the number of secondary electrons detected can be used to calculate the spacing of the crystal planes. Such a technique is described, for example, in Fibics Incorporated Tutorial, “Grain Orientation Contrast” as seen at www.fibics.com/fib/tutorials/grain-orientation-contrast/6/, which is hereby incorporated by reference.